Increases in the minimum legal drinking age (MLDA) have been shown to reduce alcohol use and related problems among young people in the US. As a policy tool, MLDA laws are effective, easy to implement and enforce and, although underage youth can still obtain alcohol through other means, generally beneficial to society. Nevertheless, economic and political pressures to lower MLDAs are strong in the US and other countries. These pressures have led to a recent decrease in the minimum purchase age (MPA) in New Zealand. One argument in support of this change is that higher MPAs lead underage youth to drink excessively in environments with little social control. Presumably lower MPAs enable young people to drink in socially controlled, safer, drinking environments (e.g., bars and restaurants). The validity of this argument is tested using a mathematical model of context specific drinking risks and a unique source of repeated alcohol surveys conducted in New Zealand before and after a reduction in the MPA in 1999 from 20 to 18 years of age. Importantly, since the primary impact of a lower MPA is to broaden the contexts in which young people can drink, the lower MPA may affect drinking risks independent of drinking levels. The model, data, and proposed statistical analyses will enable us to: (1) Measure the impacts of lowered MPA on selection of drinking contexts among underage youth, of-age youth, and adult drinkers, (2) Assess the extent to which risks related to those contexts are responsive to drinking levels in those contexts (context specific dose-response), (3) Estimate the cumulative impact of the lowered MPA on alcohol problems and their distribution across drinking contexts among youth and adults in New Zealand, (4) Examine context specific drinking risks among indigenous populations after the MPA change. Eight specific hypotheses will be tested with regard to expected impacts of the lowered MPA on drinking patterns and problems in the general population and indigenous groups in New Zealand. The short-term goal of the project is to assess the impacts of the lowered MPA upon youth drinking and context specific risks for alcohol problems in New Zealand. The public health significance of achieving this goal will be the development and application of new methods for the measurement of these risks. Since the research will be conducted using data from a foreign but comparable English-speaking country, the results of the study will generalize, within limits, to populations in the US. The research will contribute substantively to the formulation of adequate quantitative frameworks with which to assess the impacts of reductions in MLDAs. Thus, the long-term goal and public health significance of this study will be to advance the scientific foundations upon which future evaluations of the impacts of changes in MLDA laws can be based.